It depends very much on the context - what was said before this sentence. (a) 'Though' would not be used, because it would require another subordinate phrase in the sentence and would not use a comma. (b) 'Nonetheless' could be used if the sentence before implied that there were problems with the agreement and the agreement was expected despite that. "There was much disagreement between the two men on many significant issues other than bio-fuels. Nonetheless,..." (c) 'Besides' doesn't really fit. It would be used if there was at least one main item to be discussed and the bio-fuels agreement was added as a minor point. This is not the case from what I see. Also you would see it perhaps as "Besides tariff discussions, ...." (d) 'Instead' would be used if the two men were going to discuss something ELSE besides bio-fuels and changed the agenda. Again, as in (c), this is not the case.

You can use all of them. You need to refer to the context and decide which one is more suitable.

Grammatically, any of those can be suitable answers. We really need to have the sentence that comes before in order to determine which one is best for the meaning.

b) Nonetheless seems the best one to me for starting the sentence, but I agree with the below answers.